Anaerobic respiration

Anaerobic respiration is a form of respiration using electron acceptors other than oxygen. Although oxygen is not used as the final electron acceptor, the process still uses a respiratory electron transport chain; it is respiration without oxygen. In order for the electron transport chain to function, an exogenous final electron acceptor must be present to allow electrons to pass through the system. In aerobic organisms, this final electron acceptor is oxygen. Molecular oxygen is highly oxidizing and, therefore, is an excellent acceptor. In anaerobes, other less-oxidizing substances such as sulfate (SO₄^2-), nitrate (NO₃^-), or sulfur (S) are used. These terminal electron acceptors have smaller reduction potentials than O₂, meaning that less energy is released per oxidized molecule. Anaerobic respiration is, therefore, in general energetically less efficient than aerobic respiration.

Anaerobic respiration is used mainly by prokaryotes that live in environments devoid of oxygen. Many anaerobic organisms are obligate anaerobes, meaning that they can respire only using anaerobic compounds and will die in the presence of oxygen.

Comparison to fermentation

Anaerobic respiration and fermentation are two distinct forms of oxygen-independent energy metabolism. In anaerobic (and also aerobic) respiration, organisms channel electrons from an electron donor to a final electron acceptor through an electron transport chain, which converts the chemical energy into an electrochemical gradient. The energy stored in this gradient is then used in a second reaction by ATP synthase to generate ATP. In fermentation, ATP is directly synthesized from phosphorylated intermediates of metabolized compounds without the involvement of an electron transport chain. As there is no external electron acceptor in fermentation, cells have to produce their own electron acceptor to maintain their redox balance.

Ecological importance

Anaerobic respiration plays a major role in the global nitrogen, sulfur, and carbon cycles through the reduction of the oxyanions of nitrogen, sulfur, and carbon to more-reduced compounds. Dissimilatory denitrification is the main route by which biologically fixed nitrogen is returned to the atmosphere as molecular nitrogen gas. Hydrogen sulfide, a product of sulfate respiration, is a potent neurotoxin and responsible for the characteristic 'rotten egg' smell of brackish swamps. Along with volcanic hydrogen sulfide, biogenic sulfide has the capacity to precipitiate heavy metal ions from solution, leading to the deposition of sulfidic metal ores. BLAH BLAH BLAH! What a lot of information, better make note for no raisin.

Economic relevance

Dissimiltory denitrification is widely used in the removal of nitrate and nitrite from municipal wastewater. An excess of nitrate can lead to eutrophication of waterways into which treated water is released. Elevated nitrite levels in drinking water can lead to problems due to its toxicity. Denitrification converts both compounds into harmless nitrogen gas.

Methanogenesis is a form of carbonate respiration that is exploited to produce methane gas by anaerobic digestion. Biogenic methane is used as a sustainable alternative to fossil fuels. On the negative side, uncontrolled methanogenesis in landfill sites releases large volumes of methane into the atmosphere, where it acts as a powerful greenhouse gas.

Specific types of anaerobic respiration are also used to convert toxic chemicals into less-harmful molecules. For example, toxic arsenate or selenate can be reduced to less toxic compounds by various bacteria.

Examples of respiration

examples of respiration types

type	lifestyle	electron acceptor	products	Eo' [V]	example organisms
aerobic respiration	obligate and facultative aerobes	oxygen O2	H2O + CO2	+ 0.82	eukaryotes
iron reduction	facultative aerobes, obligate anaerobes	ferric iron Fe(III)	Fe(II)	+ 0.75	Geobacter, Geothermobacter, Geopsychrobacter, Pelobacter carbinolicus, P. acetylenicus, P. venetianus, Desulfuromonadales, Desulfovibrio
manganese reduction	facultative or obligate anaerobes	Mn(IV)	Mn(II)		Desulfuromonadales, Desulfovibrio
cobalt reduction	facultative or obligate anaerobes	Co(III)	Co(II)		Geobacter sulfurreducens
uranium reduction	facultative or obligate anaerobes	U(VI)	U(IV)		Geobacter metallireducens, Shewanella putrefaciens, (Desulfovibrio)
nitrate reduction (denitrification)	facultative aerobes	nitrate NO3−	nitrite NO2–	+ 0.40	Paracoccus denitrificans, E. coli
fumarate respiration	facultative aerobes	fumarate	succinate	+ 0.03	Escherichia coli
sulfate respiration	obligate anaerobes	sulfate SO42−	sulfide HS−	- 0.22	Desulfobacter latus, Desulfovibrio
methanogenesis (carbonate reduction)	methanogens	carbon dioxide CO2	methane CH4	- 0.25	Methanothrix thermophila
sulfur respiration (sulfur reduction)	facultative aerobes and obligate anaerobes	sulfur S0	sulfide HS−	- 0.27	Desulfuromonadales
acetogenesis (carbonate reduction)	acetogens	carbon dioxide CO2	acetate	- 0.30	Acetobacterium woodii
TCA reduction	facultative or obligate anaerobes	trichloroacetic acid	dichloroacetic acid		Trichlorobacter (Geobacteraceae)

See also

• Anaerobic digestion
• Anaerobic organism
• Halorespiration
• Mechanical biological treatment

Bibliography

• Lawrence, Eleanor; Holmes, Sandra (1989), Henderson's dictionary of biological terms (10th ed.), University of Michigan: Wiley, ISBN 9780470214466
• Lane, Nick (2005), Power, sex, suicide: mitochondria and the meaning of life, Oxford University Press, ISBN 9780192804815
• Sparknotes Biology Study Guide Anaerobic Respiration